Back to State Outlooks

Western Australia worms, flies and lice update - October 2018

WA WormBoss Worm Control Programs

WA WormBoss Drench Decision Guides





Albany: Brown Besier, Brown Besier Parasitology (

Across WA
Harvest is underway or starting across the state, but we are some way away from the time when sheep worms will be a lesser issue.  While ever there is some green pasture, there is a potential worm risk, especially in lambs—and with rains continuing into late October, a blowfly strike risk.

However, it is time to organise summer–autumn worm control plans, as this is the basis of worm control over the next year. It’s also an opportunity to check drench effectiveness in sheep that do need drenches at this time.

Worms in lambs

Lambs weaned a few weeks ago may have picked up a burden since a weaning drench, so doing worm egg counts to check the need for further treatment before summer is a good precaution. Ideally, if they were moved to a low–worm pasture after a drench, counts will be low and no drench will be needed, but this can’t be guaranteed.

Worm control in prime lambs is especially important, as a growth check at this time will be costly—moderate worm burdens may not be visible, but may mean the lambs are a couple of kilograms lighter than they could be. Fast-growing lambs will handle worms with little penalty, but where the feed doesn’t support ideal growth rates, worms can take a significant toll.

A drench or a worm test by 14–16 weeks of age is essential, whether or not they are weaned at that time. Leaving this until the first draft is sent off often leads to worms in the lambs, and continued contamination of the pasture with worm eggs—increasing the likelihood of worm problems.

Planning for summerautumn worm control

Reducing worm burdens in sheep when on crop stubbles or dry pastures is the cornerstone of worm control in Western Australia, but treatments need to be given at the right time, and with an effective drench.

Lambs and hoggets: Drenches in November or December are necessary for orange and white taggers, either as they go onto crop stubbles or once pastures have dried off. If harvest is delayed, and no drench has been given for 5–6 weeks, worm egg counts should be checked as even though pastures may be drying off, worm larvae can survive until really hot weather sets in.

Adult sheep: Worm burdens are generally low coming into summer, and this gives us the opportunity to manage drench resistance. The aim is to ensure a small proportion of the worm population is not exposed to drenches in summer, so some worms remain to dilute resistant worms next year, and prevent an increase in resistance.

The two main options for achieving this are:

  • Drench adult sheep in autumn, not in summer. While worm burdens are initially low, counts can rise over summer as immature worms develop, and an autumn treatment will remove these and pre–empt pasture contamination by worm eggs ahead of winter. Autumn drenches should be given in late March or early April.
  • Drench in summer onto stubble paddocks, but leave 10–20% of the mob undrenched. There are always plenty of sheep in good body condition (3.5 plus), and these won’t be affected by worms, so not drenching them will not reduce flock production. However, it benefits by leaving a small number of non–resistant worms to help reduce the development of drench resistance.

Which way to go with these options for ewes (and adult wethers)? If they are in good body condition, in most cases either option could be used, but if they have had a battle with worms during the year, or where they are well down on their normal body condition, counts may well be higher than usual. A worm egg count will quickly sort this out, and if the mob average count is around 200 eggs per gram or more, a drench is needed now (but leave some of the best-condition sheep undrenched).

Drench choice

Summer and autumn drenches should be as close to 100% effective as possible.

Recent drench resistance tests indicate that the commonly-used abamectin is often no longer fully–effective, and without checking, it should not be relied upon. Resistance is less common in the more potent moxidectin (25–30% of tests), but it should also be checked.

Resistance to the “triple combination” drenches is still uncommon, and resistance has not yet been found in Western Australia to the newer drench types, although as time goes on it will be necessary to check that they are up to the mark. Following the drench resistance management guidelines on the WormBoss website will ensure the best chance of maintaining a good range of effective drenches for many years.

Checking for drench resistance

Ideally, a full drench resistance test, where several drench types can be compared side-by-side, should be done every 2 years on lambs at weaning. (Talk to an adviser to arrange this, or check the WormBoss website.)

However, as a simple guide to drench effectiveness, checking worm egg counts before and after a drench gives an approximate result. Samples can be taken when a mob is yarded, and then from the mob in the paddock 2 weeks later.  Doing this over a few mobs and at various times of the year will allow you to check a number of drench types with little effort.

Note: If planning a drench test, make sure that mobs to be tested have not had a drench for the last 2 months or longer, as any resistant worms that survive a treatment will bias the result.


Most of the state will still be running the gauntlet with blowflies, and the risk will remain until a good spell of hot and dry weather sets in. Depending on how recently sheep were shorn, mob-wide preventative treatment may still be justified, especially in southern parts of the state.

Flystrike management can be a dilemma in prime lambs due to the meat withhold periods, and sorting out which chemicals are most appropriate is aided by the new DPIRD (WA) “Flystrike Assist”, which can be downloaded as an app, or found on the DRPID website: The FlyBoss website also has chemical use advice, and tools to help with treatment decisions based on the environment or location.

Lice treatments at shearing

A key issue is always: is a lice treatment justified?  A number of factors determine the infestation risk, and in general, the more neighbouring properties, and the more times of shearing in the year, the greater the risk.

If planning treatments, the questions are whether a particular product will be effective, and how should it be applied for best effect? Resistance by lice is widespread to some chemical classes, and care with all applications is always essential. For dipping, it is essential that sheep are wet to the skin, as break-downs can arise when shower–dipped sheep are not left in the dip for a long–enough period.

The LiceBoss website has all the information needed to help navigate the decision pathways on the need for treatment, and the most appropriate chemical choice.

Websites: See to access the WormBoss, FlyBoss and LiceBoss websites.

For October 2018 state outlooks, please follow the links below:
Back to State Outlooks